1. Field of the Invention
The present invention relates to an optical recording medium, such as an optical video disk, a compact disk, or the like, in which a reflective metallic thin film is formed through evaporation on a surface of a substrate on which information bits of sub-micrometer size are to be formed. One surface of the substrate is irradiated with light so as to read the information bits by means of reflected light to thereby transmit information, such as an image, sound, characters, figures, and the like.
2. Background of the Invention
Conventionally, an optical video disk, an example of the optical recording medium of the kind described above, is formed as shown in FIGS. 1 and 2. The method of manufacturing the recording medium will be described hereunder.
In the drawings, a substrate 1 is made of a macromolecular compound consisting of components having a polar group at their side chains, for example, acrylic resin or epoxy resin. Information bits 5 are formed on the surface of the substrate 1 by means of a stamper or the like.
A thin film 2 made of aluminum or an alloy of the same is formed through evaporation as a reflective film on the surface of the substrate 1 having the information bits 5 formed thereon so as to reflect light rays, such as laser light or the like, which enter the substrate 1 from its opposite surface.
The thus evaporation-formed thin film 2 is coated with a protective film 3. Then an adhesive 4 is applied on the protective film 3. Two of the thus prepared substrates 1 are stacked together with the respective surfaces each applied with the adhesive 4 on their joining surfaces being stuck together. Thus, an optical video disk is fabricated.
In one method, the evaporation of aluminum is performed in a high vacuum, for example, at 10.sup.-5 torr and the aluminum thin film 2 is formed at a high speed, for example, at an evaporation speed of not lower than 2 nanometers per second. This method is recommended for general use in order to provide a beautiful appearance. In fact, this method is used in commercial production.
In order to read the information bits 5 by irradiating the surface of the substrate 1 with light rays such as laser light or the like, the light rays such as laser light or the like are concentrated into a spot of about 1 micrometer in diameter to irradiate the bit surface with the light spot to thereby read the information by analyzing the reflected light from the bit surface.
If the evaporation of aluminum is performed in a high vacuum and at a high speed as described above, generally, the crystal grain size of the thin film is generally equal to or larger than 50 nanometers.
If the grain size is larger than about 50 nanometers, extremely small peelings are generated between the substrate 1 and the aluminum thin film 2 owing to time aging to thereby generate blisters 6 of substantially the same size as the information bits 5 as shown in FIG. 3.
Such blisters 6 are important in the case where information is read from an optical recording medium as described above. That is, the disk is subject to a defect that faults such as erroneous reading of proper signals occur.